Charles H. Spurgeon on Psalms 140:3

Adders' poison is under their lips. The word rendered "adder", bwvke, achsub, occurs here only; and it is perhaps impossible to determine what species is intended. As the word, in its proper signification, seems to express coiling, or bending back —an act common to most serpents —the name has perhaps no determinate reference; or it may be another name for the pethen, mentioned under Job 20.; which seems also to have been the opinion of the Seventy, as they render both words by, aspiv, and are followed by the Vulgate (aspis). As to the poison, it will be observed, that in the venomous serpents there is a gland under the eye secreting the poisonous matter, which is conveyed, in a small tube or canal, to the end of a fang which lies concealed at the roof of the mouth. This fang is moveable at the pleasure of the serpent, and is protruded when it is about to strike at an antagonist. The situation of this poison, which is, in a manner, behind the upper lip, gives great propriety to the expression, "adders' poison is under their lips." The usage of the Hebrew language renders it by no means improbable that the fang itself is called Nwvl lashon, a tongue, in the present text; and a serpent might then be said to sharpen its tongue, when, in preparing to strike, it protruded its fangs. We do not see any explanation by which a more consistent meaning may be extracted from the expression here employed. —John Kitto, in the "Pictorial Bible."